The present invention relates generally to machines for measuring and adjusting small force loads. In particular, the present invention is a machine for adjusting the gram load on head gimbal assemblies and/or suspension assemblies used in magnetic disk drives, by bending or otherwise changing the characteristics of the load beam spring.
Head gimbal assemblies are commonly used in magnetic disk drives to support magnetic heads in close proximity to moving disk surfaces. Assemblies of this type are disclosed generally in the Watrous U.S. Pat. No. 4,167,765, and typically include an air bearing head slider assembly mounted to a suspension assembly. The suspension assembly includes a base plate configured to be mounted to a positioning arm, a load beam, and a T-flexure or gimbal. The load beam is an elongated, triangularly shaped member which extends from the base plate. The gimbal is located at the load beam apex. The air bearing head slider assembly contains a magnetic head and is mounted to the gimbal. The load beam is typically manufactured by chemically etching a thin sheet of stainless steel, and mechanically forming flanges and lead clamps on the sides of the blank between the base and apex. The gimbal is also etched from stainless steel, either integrally from the apex of the load beam, or from a separate member which is welded to the load beam. During subsequent manufacturing operations the slider assembly is mounted to the gimbal and the wire leads from the magnetic head clamped to one of the load beam flanges.
High performance disk drive operation requires the air bearing head slider assembly to closely follow the moving magnetic disk surface at a constant spacing and attitude. To meet this critical requirement, the gram loading force applied to the slider assembly by the suspension assembly or load beam must be within a relatively tight specification window. In one disk drive, for example, the specification window is 7.50-7.90 grams (i.e., the upper and lower window specifications), with 7.70 grams being nominal. The gram load of the load beam is therefore adjusted after the suspension assembly is manufactured, and before the gimbal and/or head slider assembly are mounted thereto.
The assignee of the present invention has for a number of years used what is known as a "light adjust" method to adjust the gram load of load beams following their manufacture. A known property of stainless steel members such as the load beams is that the force they exert in response to attempts to bend them can be reduced (stress relieved) through exposure to thermal energy. The functional relationship between the amount of force reduction and the amount of heat to which a member is exposed can be empirically determined. The light adjust method makes use of this empirically determined relationship to "downgram" or lower the gram load on load beams that have been purposely manufactured to have an initial gram load greater than the desired specification window. Downgramming by means of the light adjust method has been found to accurately produce load beams having stable loads.
The light adjust method uses a load cell to measure the gram load of a load beam clamped to a test fixture. A computer controlled actuator moves the load cell with respect to the load beam. As the load cell measuring arm is engaged by the load beam, the computer monitors the measured gram load as a function of time. In practice, the measured gram load quickly slews (i.e., rises) toward its current actual value. When the measured gram load reaches the upper window specification, the computer actuates or turns on a high intensity lamp to apply heat to the load beam. Since the applied heat reduces the actual gram load of the beam, the measured gram load quickly peaks at a point above the upper window specification. Continued application of heat causes the measured gram load to decrease with time. The computer deactuates or turns off the lamp when the measured gram load has decreased to a predetermined set point, typically a load between the nominal gram load and the lower window specification. Once the lamp has been turned off, the measured decrease in gram load quickly slows and reaches its minimum value (often at a gram load below the lower window specification) as the heat in the load beam dissipates. However, as the load beam continues to cool, the measured gram load increases and stabilizes at an equilibrium or final load value that is preferably well within the specification window, and ideally close to the nominal specification. The final gram load of the load beam is also measured following the light adjust procedure. This measurement is used by the computer to continually update the stored model (e.g., the setpoint) of the functional relationship between the amount of heat applied and the gram load reduction to optimize the accuracy of the results obtained by the light adjust procedure.
The assignee of the present invention also uses a computer controlled mechanical bending method to adjust the gram load on load beams. The mechanical bending method makes use of an empirically determined relationship between the amount that a load beam is mechanically bent and the associated change in gram load. For the range of gram load adjustments that are typically performed by this technique, a simple linear regression line has been found to accurately describe this relationship. In practice, this technique is implemented by a computer coupled to a stepper motordriven bending member and a load cell. A model of the relationship between changes in gram load and the number of motor steps (i.e., the associated amount or extent of bending required) is stored in the computer. After the actual load on the load beam is measured by the load cell, the computer calculates the required load correction (i.e., the difference between the measured and desired loads). The computer then accesses the model as a function of the required correction to determine the number of motor steps needed to achieve the required load correction, and actuates the stepper motor accordingly. Once the load beam has been bent, the actual load is again measured and used to update the model. Measured data from a given number of the most recently executed mechanical bends is used to recompute the regression line data prior to the execution of the next mechanical bend. Although this bending technique can be used to both increase and decrease gram loads, it does not offer sufficient long term stability.
The air bearing head slider assemblies are mounted to the gimbal, and the head leads clamped to the load beam, after the gram load of the load beam has been initially set using methods such as those described above. Unfortunately, the mechanical handling and assembly procedures involved in this manufacturing operation sometimes force the gram loads of the assembled head gimbal assemblies beyond the specification window. Since the gram load specification is so critical to proper disk drive operation, these out-of-specification head gimbal assemblies cannot be used.
Head gimbal assembly manufacturing is by nature a high volume, low margin business. To date, it has not been commercially feasible to "regram" (i.e., readjust the gram load) of out-of-specification head gimbal assemblies by means of the techniques described above. Nonetheless, the waste associated with the unusable out-of-specification assemblies is relatively great. With the development of higher performance disk drives requiring head gimbal assemblies with even tighter load window specifications, it is likely that the number of out-of-specification assemblies, and therefore the waste, will continue to rise.
A microprocessor controlled manufacturing system for load support arms is disclosed in the Smith Et. al. U.S. Pat. No. 4,063,567. This system includes a waling beam transport system for moving load beams extending from frets between several stations. A radius forming station mechanically bends the load support arms to increase gram loads. A thermal adjust station is operated in a manner similar to the light adjust method described above to reduce gram loads. The load beams are cut from the fret at a shear station after they have been processed at the radius forming and thermal adjust stations. This manufacturing system is not, however, adapted for readjusting gram loads after the load beams have been assembled into gimbal assemblies. The system is also relatively complicated.
It is evident that there is a need for a machine capable of economically readjusting the gram loads on suspension assemblies and head gimbal assemblies. The machine must of course be capable of accurately adjusting the gram loads. To be commercially viable, the machine must also be relatively easy to operate and capable of quickly performing the regram operations.